1. Field of the Invention
The present invention relates to an improvement on a focus detecting device for performing focus detection of the passive type using phase-difference detection and an automatic focusing camera having the focus detecting device.
2. Description of Related Art
In passive-type automatic focusing single-lens reflex cameras, at present, a main trend is to use the so-called phase-difference detecting AF method in which the amount of defocus is detected from the amount of deviation of relative positions of two images on an AF sensor formed by light fluxes having passed through respective different areas of a photographic lens. A general example of such a camera will be explained by using the electrical block diagram shown in FIG. 1.
In FIG. 1, reference numeral 1 denotes a sequence controller for managing the sequence of the camera. Reference numeral 2 denotes a focus detecting device of the phase-difference detection type using a pair of line sensors (being assumed to receive an image "A" and an image "B") for measuring the distance to an object or for detecting a focusing state. Reference numeral 3 denotes an interchangeable photographic lens capable of being driven for focusing according to a result of focus detection obtained by the sequence controller 1 and the focus detecting device 2. Reference numeral 4 denotes a light measuring device for measuring the luminance of an object. Reference numeral 5 denotes a film exposure device for driving a shutter, etc., on the basis of the output of the light measuring device 4. Reference numeral 6 denotes a film transport device for transporting a photographic film. Reference numeral 7 denotes an information display device for displaying information such as a value counted by a film counter. Reference numeral 8 denotes a sound producing device, such as a buzzer, for producing an in-focus confirmation sound or a self-timer sound. Reference numeral 9 denotes a switch (SW1) which is operable for starting the focus detection and the light measurement and is turned on by the first stroke of a release button. Reference numeral 10 denotes a switch (SW2) which is operable for starting the exposure of film and, then, starting the winding of film and is turned on by the second stroke of the release button. Reference numeral 11 denotes a battery for supplying electric power to the camera.
FIG. 18 is a flow chart showing the action of the camera having the construction shown in FIG. 1, including the turning-on of the switch SW1 up to the release operation.
Referring to FIG. 18, first, a check is made to find if the switch SWl is turned on (S701). If so, an automatic focusing operation is started. That is, an object image, which is a target for focus detection, is accumulated by the focus detecting device 2 (S702), and, then, the accumulated object image is read out (S703). Next, from the amounts of correlation of outputs of two line sensors which are paired, a phase difference on the line sensors is obtained (S704). Here, due to any errors in manufacturing cameras, the phase difference does not necessarily coincide with an ideal value in such a state that focus is adjusted on the surface of film in the camera. Such an amount of deviation of the phase difference is generally obtained at the adjustment step of each camera during manufacturing of cameras, and is stored in a memory of each camera.
After the phase difference on the line sensors is obtained in the above step S704, next, the correction of the deviation of the phase difference (phase-difference correction) is performed (S705). The thus-obtained corrected phase difference is converted into an amount of defocus (defocus amount) on the film surface (S706). Then, a check is made to find if the defocus amount is within an in-focus width (the amount of tolerance according to which an in-focus state can be considered to be obtained) (S707). If the defocus amount is not found to be within the in-focus width (i.e., if an in-focus state is not considered to be obtained), the photographic lens 3 is driven as much as the defocus amount (S708). Then, the flow returns to step S701. If, in the step S701, the switch SW1 remains turned on, the focus detecting operation is performed again (S702 to S706) After that, if the defocus amount is found in the step S707 to be within the in-focus width, a check is made for the states of the switches SW1 and SW2 (S709 and S710). If both the switches SW1 and SW2 are turned on, a release operation, including an exposure operation and a film winding operation, is performed (S711), and the sequence of the camera comes to an end.
In the conventional camera having the abovedescribed construction, in a case where an image formed on the line sensor is a bar chart which has such a thin width that is close to a pixel pitch of the line sensor, as shown in FIG. 19, output waveforms of the line sensors are greatly varied depending on the portions at which the bar charts are formed on the line sensors, so that, in some cases, a result of the correlation computation has errors with respect to the actual phase difference. In the case of FIG. 19, although the image "A" is the same in an optical sense (has the same width) as the image "B", the output waveform of the line sensor on the side of the image "A" has a lower peak than that of the output waveform of the line sensor on the side of the image "B", as shown in the lower part of FIG. 19, because the image "A" extends over two pixels of the line sensor. Accordingly, the result of correlation computation comes to indicate a value different from the actual phase difference. More particularly, if a dead zone part of each pixel of the line sensor is so wide as to be unignorable with respect to the whole pixel, such a difference would increase, resulting in a defocus state on the film surface. In the past, it has been impossible to take any measures over the defocus caused by errors due to such a thin-line chart.